A strike currently affecting 26 downtown Chicago hotels is the first broad hotel strike the city has seen, according the union that called for it.

It started Friday and, the union says, will go on until the hotels agree to its demands.

Here are five things to know.

Who is striking?

Hotel workers — housekeepers, doormen, cooks, bartenders, room service attendants and more — whose contracts expired Aug. 31 are on strike. The union that represents the workers, UNITE HERE Local 1, says that 6,000 workers are covered by those expired contracts, though it isn’t clear how many people have actually walked off the job.

Which hotels are affected?

Workers are on strike at 26 downtown Chicago hotels, including the JW Mariott, the Palmer House Hilton, the Hyatt Regency and the Sheraton Grand. The strike began Friday at 25 hotels and workers at a 26th hotel, the Cambria Chicago Magnificent Mile, walked off the job Monday. The expired contracts cover 30 hotels, and the four not yet striking are in labor disputes and could join, the union says. A list of affected hotels is at chicagohotelstrike.org.

Why are they striking?

The primary demand of the striking workers is to include a guarantee of year-round health insurance in their new contracts. Currently, many hotel employees lose their health insurance when hotels temporarily lay people off during the slow season, generally October through March, and have coverage reinstated when they are brought back to work when the weather warms. Though workers with seniority are employed year-round and get insurance year-round, the union wants all employees, regardless of tenure, to have uninterrupted health insurance, even during the slow months when they are not working at the hotels.

How does this get resolved?

The union negotiates contracts with each employer separately. So theoretically, if an agreement is reached with one of the hotels the workers from that hotel could cease to strike. But labor expert Bob Bruno said that the most efficient and effective strategy is to get one of the larger employers to establish a baseline agreement on the issue that other hotels in the city will follow.

“The incentive (for the union) is to create a standard for the industry,” said Bruno, labor professor at the University of Illinois at Urbana-Champaign. “And the employers gain some value on that too because they know they’re not competing on the basis of cost.”

Three major hotel groups comprise the bulk of the hotels where workers are striking. Hilton Hotels and Resorts manages the Palmer House, the DoubleTree Magnificent Mile, Hilton Chicago and the Drake hotel. Hyatt Hotels manages the Hyatt Regency Chicago, Hyatt Recency McCormick and the Park Hyatt. Marriott International’s portfolio includes the Westin River North, Westin Michigan Avenue, W Chicago Lakeshore, W Chicago City Center, JW Marriott and the Sheraton Grand.

What is the response from the hotels?

Hotels have been assuring guests that they are open for business and will continue to provide excellent service. But some guests have reported eight-hour waits to check in, long lines at breakfast and having to replenish towels themselves because there are not enough people to clean rooms. Managers have said they are rolling up their sleeves to empty out garbage bins and refill soaps.

Hotels have expressed frustration that the strike came so early in contract negotiations. Marriott, for example, said the union is still in the process of making its initial bargaining proposal.

“There is nothing about the current state of the negotiations or the longstanding and productive bargaining relationship between Marriott International and UNITE HERE that suggests that a strike is warranted or necessary,” Marriott said in a statement. “The parties are not at an impasse on any issue.”

Marriott, Hyatt and Hilton have all said they continue to bargain in good faith.